There are a great variety of injection molding machines which have been developed over the years. These machines are adapted for manufacture of numerous products including various types of molded rubber parts and rubber/metal components, shoe soles or shoe parts, and multi component parts and hollow bodies. Such injection molding machines have been adapted not only for forming extensive numbers of parts of molded rubber and other elastomers but also for thermoplastic, polyurethane and other plastic components.
In general, these machines have in common the injection of a measured shot of elastomer or other material into a mold cavity where the material sets or is cured in conformance with the mold configuration. Thereafter products which are manufactured in an injection molding machine are removed from the mold, the injection molding machine is recharged with another shot of molding material and the material is again discharged into the same or a different mold cavity. The art is highly refined with respect to the heating or cooling of the molding material being supplied, the application of molding pressures and temperatures, and related operations to optimize the speed and reliability of the equipment and process for the particular application by way of the type of product and the constituents of the molding material.
The transfer and storage of the molding material from an external source until its injection into the mold has been handled in a number of different ways. In general, for products involving short molding times and rapid rates of injection, it is common to employ what is termed a reciprocating screw injection device wherein a screw aligned axially of the nozzle transfers the molding material to a chamber between the screw and the nozzle and the screw reciprocates for the discharge of the molding material from the chamber and to recharge the chamber with molding material prior to a subsequent injection stroke. In applications involving longer production intervals, as for example necessitated by higher temperature and pressure requirements, it is common to employ an injection plunger with a feedscrew supplying molding material to a chamber from which the material is injected into a mold. Machines of this type generally have the plunger operating in a barrel in which the chamber is formed. The molding material is normally introduced into the chamber through an aperture in the barrel located at the end of the chamber proximate the nozzle. This location permits the feeding of the molding material into the chamber to effect withdrawal of the plunger during the loading of the chamber with a shot of molding material. As a result, the plunger can carry out its reciprocating motion through the actuation of a relatively noncomplex single stroke cylinder attached thereto.
These injection plunger feedscrew loading machines have been subject to certain disadvantages or limitations in the form in which they are constructed in industrial use. Due to the location of introduction of the molding material into the chamber the first molding material engaging the plunger becomes the last molding material to be ejected from the chamber. In some instances the chamber is elongated below the point of introduction of the molding material or the nozzle may be substantially elongated to extend through a bolster and platen of an injection molding machine. As a result portions of a shot of the molding material may remain in the chamber or in the nozzle bore for an injection and curing cycle prior to being injected into the mold. In such instances this molding material may assume highly different flow and curing characteristics due to the fact that it may have been subjected to a precure or extent of curing due to heat transfer from the mold and platen through the nozzle and the barrel surrounding the chamber. These characteristics can readily result in products having nonuniform characteristics or even structural defects which may result in a substantial incidence of scrap or waste.
An additional disadvantage of these injection plunger feedscrew loading machines is that, with the location of the introduction of molding material at the extremity of the chamber proximate the nozzle, the common upright type injection molding machine normally has the plunger and the actuating cylinder therefor positioned atop the upper bolster of the machine. With machines capable of ejecting shots of any substantial volume, the height of the machines may reach such substantial proportions such that they may be installed in only a limited number of facilities where there is a substantial ceiling height.